In conventional practice, polymers--whether resins or addition or condensation polymers--are made according to a sequence and conditions programmed for charging the monomers and introducing catalysts, promoters, initiators and chain extenders, emulsifiers, eliminating volatiles and controlling experimental conditions (temperature, agitation, viscosity and reaction index, etc.).
Polymers are obtained from the polymerizing units in the form of beads, etc. In many cases, the polymerizing units are combined with equipment, in which the formed polymerized materials are processed after the introduction of the corresponding additives and fillers in order to obtain articles shaped by spinning, extrusion, molding etc.
In other cases, the polymerized materials processed, as applicable, by the incorporation of additives--antioxidants, plasticizers, etc.--are stored as raw material for subsequent manufacturing processes, as well as in the form of semi-finished products such as sheets, bars, etc., processed at a later date to form film by blowing, foam-type materials, etc. In these cases, where the finished products require prior formulation of the polymerized material, it is common practice to introduce fillers, additives and reinforcing material that can be selected in terms of the combinations of polymer and the desired effects.
This practice of postponing the filling and/or the reinforcing of the polymerized materials imposes additional costs on the processes used to formulate and manufacture finished polymerized articles, since the introduction of reinforcing materials after the fact requires reprocessing of the fusion-polymerized material (with the optional inclusion of plasticizers).
At the same time, the later inclusion of the reinforcing material requires careful and controlled homogenization of the mixture (polymerized material+reinforcing material) in mixing operations with an energy requirement that is a function of the viscosity of the mixture (which also depends on the temperature, the molecular weight of the polymer and on the polymer/reinforcing material weight ratio). Clearly, the solution to these drawbacks seems evident: raising the temperature, which usually is counterproductive because of the effect of temperature on the depolymerization and decomposition of the polymerized material.
The technology concerning the production of thermoplastic polymers reinforced by conventional means is adequately described in Patent No. 677,296 awarded to Giulini Chemic GmbH on Mar. 4, 1985, included herein by reference. This patent deals with the reinforcing of polymer materials (especially epsilon caprolactone polymers, elastomer polyurethanes, modified polyolefins, etc.).
Certain types of thermoplastic materials are used in the footwear industry to form and retain the structure of the toe and heel of the shoe, in fusion with reinforcing fillers in particles 50 to 500 microns in diameter. Thus, reinforced thermoplastic sheets are obtained, from which pieces are then cut to reinforce the toe and heel of footwear in general.
The principal characteristic of these materials is that they must be sufficiently rigid and at the same time elastic so as to give them the necessary structure and retain the memory of the form, despite distortions produced by wear.
At the same time, it is desirable that, during the manufacturing of the footwear, the material can be shaped, copying the last, and acting like a plastic material above a specified temperature that is called the activation temperature, then becoming elastic again and retaining the memory of the last once it cools. This is called the thermoplastic feature characteristic.
It is desirable for the material to be able to adhere to the reinforcing materials; in some cases, one side is sufficient, but in other cases it is appropriate that it be able to adhere to both the lining and cap side. The most appropriate feature in the manufacture of footwear is for the material to have heat-sensitive adhesive qualities so that, when heated above the activation temperature, the adhesive melts and penetrates the other material superficially and continues to adhere after it has cooled down. These are the characteristics of thermoadhesion.
In some cases, it is appropriate for the adhesive, as it melts, to liquefy more on one face than on the other. This is the case, for example when a dressed leather is to be glued to one face and a very fine fabric to the other. In some low-cost shoe designs, the toecap material does not need adhesive on both sides, because no linings are put in since they are not visible. In the case of low-cost, unlined, shoes, where the heel reinforcement material is visible, materials can be used that include the lining material in the reinforcement, laminated on the face that is visible so as to eliminate the lining in the construction of the shoe.
The best-known materials are produced by impregnating a fabric or non-woven fleece with an emulsion of thermoplastic polymer, usually carboxylated butadiene styrene. This polymer softens and becomes plastic at a temperature of 70.degree. to 90.degree. C., but does not liquefy; for adhesion, it is necessary to laminate on a light coating of heat-sensitive adhesive. The material most commonly used as a heat-sensitive adhesive is a blend of a polyethylene vinyl acetate copolymer with resins and other additives. Also used as adhesives are cyclic ester polymers or copolymers and blends of these known polymers with polycaprolactone, in accordance with U.S. Pat. No. 3,778,251 and Canadian Patent 1,027,838. The advantage of using this polymer is that it not only serves as an adhesive, but also helps provide elastic rigidity to the reinforcing material, since the polycaprolactone has the particular property of melting at a temperature below 60.degree. C., and once cooled, of crystallizing below 50.degree. C., to give excellent elastic rigidity to the material.
Another material widely used to reinforce toecaps is a polyamide copoylmer (for example, U.S. Pat. No. 4,122,229), which is applied in molten form; in this case, the adhesive itself is also the reinforcing material.
Another material used consists of a blend of ground plastic particles and polycaprolactone in accordance with the German patent cited. In this case, the ground plastic is used as a filler and polycaprolactone as the agglutinant. The agglutinant of the reinforcing material sometimes also serves as an adhesive.
Other materials used are ionomer sheet extrusions; these polymers have the particular property of melting at low temperatures of 80.degree.-90.degree. C.; once cooled, they have excellent elastic rigidity and a high viscosity, which therefore prevents direct adhesion and makes the lamination of heat-sensitive layers of adhesive necessary.
In addition, multilaminated materials made of combinations of the above-mentioned woven and non-woven materials, impregnated or not, and sheets of polymers extruded in fine layers are also used.
In all cases, then, thermoplastic polymers are used for the production of thermoplastic reinforcing materials. In both the patented cases and the not patented cases, the thermoplastic polymers are used in the polymerized state. This means that, for processing them, they have to be transformed physically by supplying energy to melt them.
The molten polymers have a viscosity that increases with the molecular weight and decreases as the temperature increases. For the processing, it is desirable for the viscosity to be low, so that polymers can be worked with greater ease; in the case of compound materials, the low viscosity makes it possible to wet the fibers or particles of filler. It is also known that the binding power and strength of polymers is proportional to their molecular weight, the polymers with the greatest binding power being the most appropriate for reinforcing materials. The solution is to raise the processing temperature which degrades the polymers and entails an expenditure of energy, since the polymers have to be cooled again after the processing.
At the same time, the polymers utilized have molecular weights and softening temperatures determined by their manufacturers and, while there are different grades, it is not possible to regulate them to meet the optimum requirements of each application.
These aspects of the technology relating to the production and conventional application of polymers and their effect on the practices and costs of the footwear industry, particularly as regards the manufacture of the reinforced polymer components used in modern footwear (heel pieces, etc.) have been examined and reviewed by the Applicant. This technology is adequately discussed and illustrated in German patent 677,296 awarded to Giulini Chemic GmbH, already referred to earlier.